FREQUENCY MODULATOR AND FM TRANSMISSION CIRCUIT USING THE SAME

• US 20100213916A1
• Filed: 12/05/2006
• Published: 08/26/2010
• Est. Priority Date: 12/06/2005
• Status: Active Grant

• Current Assignee: ROHM Co., Ltd.
• Original Assignee: ROHM Co., Ltd.
• Inventors: Sagara, Takeshi, Komori, Hirofumi
• Granted Patent: US 7,936,228 B2
• Time in Patent Office: Days
• US Class Current: 323/293
• CPC Class Codes: H03C 3/095 applying frequency modulation to the loop in front of the voltage controlled oscillator
• CPC Class Codes: H04H 20/48 for FM stereophonic broadcast systems

Abstract

An input signal is input via a first resistor to an inverting input terminal of an operational amplifier. A second resistor is provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier. A control voltage Vcnt output from the operational amplifier is input to a VCO. A frequency divider frequency-divides an output signal Sout of the VCO. A phase comparator compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference. A loop filter removes a high-frequency component of an output voltage Vcp of the phase comparator and outputs the voltage to a non-inverting input terminal of the operational amplifier.

Claims

1. A frequency modulator comprising:

an operational amplifier having an inverting input terminal to which an input signal is input via a first resistor;

a second resistor provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier;

a voltage controlled oscillator to which an output signal from the operational amplifier is input;

a frequency divider which frequency-divides an output signal from the voltage controlled oscillator;

a phase comparator which compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference; and

a filter which removes a high-frequency component of an output voltage from the phase comparator and outputs the voltage to a non-inverting input terminal of the operational amplifier.

2. The frequency modulator according to claim 1, wherein

a resistance of the first resistor is set higher than a resistance of the second resistor.

3. The frequency modulator according to claim 1, wherein

at least one of the first and second resistor is a variable resistor.

4. The frequency modulator according to claim 1, wherein

the operational amplifier is a rail amplifier.

5. The frequency modulator according to claim 1, wherein

the filter is a passive filter.

6. The frequency modulator according to claim 1, wherein

the input signal is a stereo composite signal.

7. The frequency modulator according to claim 1, wherein

the frequency modulator is integrated on a single semiconductor substrate.

8. An FM transmission circuit comprising:

a stereo modulating unit which converts an audio signal into a stereo composite signal;

the frequency modulator according to claim 1, which performs frequency modulation based on the stereo composite signal output from the stereo modulating unit and outputs the modulated signal; and

an amplifier which amplifies the modulated signal output from the frequency modulator.

9. A stereo modulator which generates a stereo composite signal based on a stereo signal, a subcarrier wave, and a pilot signal, the stereo modulator comprising:

a separation adjustment circuit which includes a delay circuit that delays an input signal by the time length, in which a predetermined clock signal is counted for n cycle(s) (n is a counting number), and inputs one of the subcarrier wave and the pilot signal to the delay circuit to be delayed;

a multiplexer which multiplexes the subcarrier wave output from the separation adjustment circuit and the stereo signal; and

a pilot signal multiplexing unit which multiplexes the pilot signal output from the separation adjustment circuit and an output signal from the multiplexer.

10. The stereo modulator according to claim 9, wherein

the number of cycles n counted by the delay circuit of the separation adjustment circuit is externally controllable.

11. The stereo modulator according to claim 9, wherein

the delay circuit of the separation adjustment circuit includes a first shift register that operates according to a clock signal, and outputs a signal n-bit shifted by the first shift register.

12. The stereo modulator according to claim 9, wherein

the delay circuit of the separation adjustment circuit includes:

a first shift register capable of shifting m bits at the maximum (m is an integral number that satisfies m≧

n; and

a selecting unit which selects an n-bit shifted signal from the first shift register and outputs the signal.

13. The stereo modulator according to claim 9, wherein

the separation adjustment circuit further includes a switch which switches either one of the subcarrier wave and the pilot signal is input to the delay circuit.

14. The stereo modulator according to claim 11, wherein

the separation adjustment circuit further includes a one-bit second shift register which delays an input signal by one clock cycle of the clock signal, and inputs the other of the subcarrier wave and the pilot signal to the second shift register to be delayed.

15. The stereo modulator according to claim 14, wherein

the separation adjustment circuit further includes a switch which switches either one of the subcarrier wave and the pilot signal is input to the first shift register and the second shift register, respectively.

16. The stereo modulator according to claim 9, wherein

the clock signal is a signal that shares the common origin with a reference clock signal used for generating the subcarrier wave and the pilot signal.

17. The stereo modulator according to claim 9, wherein

the stereo modulator is integrated on a single semiconductor substrate.

18. An FM transmission circuit comprising:

the stereo modulator according to claim 9, which converts an audio signal into a stereo composite signal;

a frequency modulator which outputs a modulated signal frequency-modulated using the stereo composite signal output from the stereo modulator; and

a power amplifier which amplifies the modulated signal generated by the frequency modulator.

19. The FM transmission circuit according to claim 18, wherein

the stereo modulator, the frequency modulator, and the power amplifier are integrated on a single semiconductor substrate.

20. An FM transmission circuit comprising:

a variable gain amplifier which amplifies an input signal so as to adjust an amplitude thereof;

a direct-modulation frequency modulator which uses an output signal from the variable gain amplifier as a modulating signal and generates a modulated signal frequency-modulated based on the modulating signal; and

a gain control unit which sets a gain of the variable gain amplifier depending on a frequency of the modulated signal generated by the frequency modulator.

21. The FM transmission circuit according to claim 20, wherein

the gain control unit sets the gain of the variable gain amplifier so as to correct frequency dependency of modulation sensitivity of the frequency modulator.

22. The FM transmission circuit according to claim 20, wherein

the gain control unit includes a memory which stores a table showing a relationship between the frequency of the modulated signal and the gain of the variable gain amplifier.

23. The FM transmission circuit according to claim 20, wherein

the frequency modulator includes:

a voltage controlled oscillator;

a frequency divider which frequency-divides an output signal from the voltage controlled oscillator;

a phase comparator which compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference;

a filter which removes a high-frequency component of an output voltage from the phase comparator; and

a multiplexing circuit which multiplexes the output signal from the variable gain amplifier with an output signal from the filter.

24. The FM transmission circuit according to claim 23, wherein

the multiplexing circuit includes:

an operational amplifier having a non-inverting input terminal to which the output signal from the filter is input;

a first resistor having one end connected to an inverting input terminal of the operational amplifier and the other end to which the input signal is input; and

a second resistor provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier.

25. The FM transmission circuit according to claim 20, wherein

the input signal is a stereo composite signal.

26. The FM transmission circuit according to claim 20, wherein

the FM transmission circuit is integrated on a single semiconductor substrate.

27. The FM transmission circuit according to claim 26, further comprising:

a stereo modulating unit which converts an audio signal into a stereo composite signal and outputs the signal to the variable gain amplifier; and

a power amplifier which amplifies the modulated signal generated by the frequency modulator, wherein

the FM transmission circuit is integrated on a single semiconductor substrate.

28. An FM transmission circuit comprising:

a direct-modulation frequency modulator which generates a modulated signal frequency-modulated based on a modulating signal;

a power amplifier which amplifies the modulated signal generated by the frequency modulator; and

a gain control unit which sets a gain of the power amplifier depending on a frequency of the modulated signal generated by the frequency modulator.

29. The FM transmission circuit according to claim 28, wherein

the gain control unit sets the gain of the power amplifier depending on the frequency of the modulated signal so that power of a high-frequency signal output from the power amplifier may be constant.

30. The FM transmission circuit according to claim 28, wherein

the gain control unit includes a memory which stores a table showing a relationship between the frequency of the modulated signal and the gain of the power amplifier.

31. The FM transmission circuit according to claim 28, wherein

the frequency modulator includes:

a voltage controlled oscillator;

a frequency divider which frequency-divides an output signal from the voltage controlled oscillator;

a phase comparator which compares an output signal from the frequency divider with a reference clock signal and outputs a voltage according to a phase difference;

a filter which removes a high-frequency component of an output voltage from the phase comparator; and

a multiplexing circuit which multiplexes the modulating signal with an output signal from the filter.

32. The FM transmission circuit according to claim 31, wherein

the multiplexing circuit includes:

an operational amplifier having a non-inverting input terminal to which the output signal from the filter is input;

a first resistor having one end connected to an inverting input terminal of the operational amplifier and the other end to which the modulating signal is input; and

a second resistor provided on a feedback path between an output terminal and the inverting input terminal of the operational amplifier.

33. The FM transmission circuit according to claim 28, wherein

the modulating signal is a stereo composite signal.

34. The FM transmission circuit according to claim 28, the FM transmission circuit is integrated on a single semiconductor substrate.

35. The FM transmission circuit according to claim 34, further comprising:

a stereo modulating unit which converts an audio signal into a stereo composite signal and outputs the signal to the frequency modulator; and

a power amplifier which amplifies the modulated signal generated by the frequency modulator, wherein

the FM transmission circuit is integrated on a single semiconductor substrate.

36. A voltage generating circuit which divides a power supply voltage applied to a power supply terminal and a ground voltage applied to a ground terminal and outputs a divided voltage from an output terminal, comprising:

a first voltage divider circuit including first and second resistors connected in series between the power supply terminal and the ground terminal, a connection point between the two resistors is connected to the output terminal;

an output capacitor provided between the output terminal and the ground terminal;

a second voltage divider circuit including third and fourth resistors connected in series between the power supply terminal and the ground terminal; and

a charging circuit which becomes active and supplies electric current to the output capacitor when a voltage at a connection point between the third and fourth resistors is higher than a voltage at the output terminal.

37. The voltage generating circuit according to claim 36, wherein

the charging circuit includes:

a first switch connected in series between the power supply terminal and the ground terminal; and

a first comparator which compares the voltage at the connection point between the third and fourth resistors with the voltage at the output terminal, and

the first switch is turned on and off according to an output signal from the first comparator.

38. The voltage generating circuit according to claim 37, wherein

the charging circuit further includes a fifth resistor connected in series to the first switch.

39. The voltage generating circuit according to claim 38, wherein

a resistance of the fifth resistor is set in a range from 1/1000 times to 1/10 times resistances of the first, second, third, and fourth resistors.

40. The voltage generating circuit according to claim 37, wherein

the first comparator has an input offset voltage.

41. The voltage generating circuit according to claim 36, further comprising a discharging circuit which becomes active and draws current from the output capacitor when the voltage at the connection point between the third and fourth resistors is lower than the voltage at the output terminal.

42. The voltage generating circuit according to claim 41, wherein

the discharging circuit includes:

a second switch connected in series between the ground terminal and the output terminal; and

a second comparator which compares the voltage at the connection point between the third and fourth resistors with the voltage at the output terminal, and

the second switch is turned on and off according to an output signal from the second comparator.

43. The voltage generating circuit according to claim 42, wherein

the discharging circuit further includes a sixth resistor connected in series to the second switch.

44. The voltage generating circuit according to claim 43, wherein

a resistance of the sixth resistor is set in a range from 1/1000 times to 1/10 times the resistances of the first, second, third, and fourth resistors.

45. The voltage generating circuit according to claim 42, wherein

the second comparator has an input offset voltage.

46. The voltage generating circuit according to claim 36, wherein

the first resistor and the second resistor of the first voltage divider circuit and the third resistor and the fourth resistor of the second voltage divider circuit are formed by pairing, respectively.

47. A signal processing circuit comprising the voltage generating circuit according to claim 36, wherein

the signal processing circuit performs predetermined signal processing using a voltage output from the voltage generating circuit as a reference voltage.

48. The signal processing circuit according to claim 47, further comprising:

a stereo modulator which converts an audio signal into a stereo composite signal;

a frequency modulator which generates a modulated signal frequency-modulated using the stereo composite signal output from the stereo modulator; and

a power amplifier which amplifies the modulated signal generated by the frequency modulator, wherein

at least one of the stereo modulator and the frequency modulator operates according to a voltage output from the voltage generating circuit.

49. The signal processing circuit according to claim 47, wherein

the signal processing circuit is integrated on a single semiconductor substrate.